it possible to work this thing on eight speakers ?
Good question. I've been thinking about just that same thing. The nice thing about this xy_to_amp patch is that when you pan all the way to R/L/F/Back, it all goes to just one speaker. And when you pan all the way (.707,.707) in between two speakers, your opposite speakers all have value of zero. You want to have the same thing for 8 (or any number of speakers)...
As for the math, just draw a little picture of a circle, radius .5, with a variable center location, and try some different positions for the center.
BTW, I have only only two speakers, so I have no idea whether or not it sounds good
Chuck